Passive intrusion detector

ABSTRACT

A passive intrusion detector is described with two stationary detectors or detector elements arranged side by side and stationary optics which image a target onto the plane of the detector. The output of the two detectors or detector elements, which are in opposition, is processed in electronic circuits involving differentiation so that there will be no final output unless the image of the target moves onto and off a detector or, in one modification, from one detector to the other. The electronics can also indicate the direction of the movement. The detector is aimed at an area where an intruder may enter, for example, a path through the jungle, and signals by the infrared radiation of the target that moves into the area or across it actuate an alarm.



1. A purely passive device for detection of moving intruders by self-radiation in the infrared, comprising in combination, a. single stationary infrared-detecting system having two infrared-detecting elements connected in output opposition and located adjacent to each other in substantially the same plane, b. a single, unitary stationary optical means for imaging a target onto the plane of the detecting elements whereby the image of the moving target in the field of view of the optics does not move unless the target does, in which case, however, the image of the target will move across one or both detecting elements, c. electronic processing and amplifying circuits coupled to the output of the detecting elements and including a differentiating circuit, the differentiating circuit having a time constant so that it responds to frequencies relatively slow compared to low audio frequencies, whereby a target image moving slowly across at least one detecting element produces an output signal, and d. alarm means actuated by said signal.
 2. A device according to claim 1 in which the differentiating circuit responds to frequencies from 0.02 to 20 c.p.s.
 3. A device according to claim 2 in which the differentiating circuit responds to frequencies from 0.2 to 2 c.p.s.
 4. A device according to claim 1 in which the electronic processing and amplifying circuits produce pulses of opposite polarity depending on which detector the target is crossing, and the alarm means are distinguishable and are actuatable by the pulses of different polarities.
 5. A device according to claim 4 in which means are provided actuated by the pulses in the output of the processing circuits for actuating one alarm when an intruder moves across the field of view in one direction and another alarm when the movement is in the opposite direction.
 6. A device according to claim 5 in which the means for actuating the alarm comprise one monostable multivibrator and an AND-gate for one alarm actuation, and a second monostable multivibrator and AND-gate for the second alarm, the multivibrators and AND-gates being connected so that one pair is actuated by a pulse sequence from an intruder moving in from one direction and the other from the opposite direction.
 7. A device according to claim 6 in which the multivibrators are provided with short-circuiting switches whereby one alarm is actuated by a target image intruding on one detector and the other alarm on intrusion on the other detector regardless of whether the target image, after intruding on one detector, intrudes on the other in sequence.
 8. A multiple field of view device according to claim 1 in which a reflecting system having a substantially flat mirror and a curved mirror is mounted at substantially 45* to the optical axis of the imaging means, the flat mirror reflecting onto one portion of the entrance pupil of the imaging means and the curved mirror on the other, the ratio of radius of curvature of the curved mirror to imaging means entrance aperture being less than 5 and not less than about 1.75, and the radius of curvature of the substantially flat mirror being at least 10 times the radius of curvature of the curved mirror.
 9. A multiple field of view device according to claim 2 in which a reflecting system having a substantially flat mirror and a curved mirror is mounted at substantially 45* to the optical axis of the imaging means, the flat mirror reflecting onto one portion of the entrance pupil of the imaging means and the curved mirror on the other, ratio of radius of curvature of the curved mirror to imaging means entrance aperture being less tHan 5 and not less than about 1.75, and the radius of curvature of the substantially flat mirror being at least 10 times the radius of curvature of the curved mirror.
 10. A multiple field of view device according to claim 3 in which a reflecting system having a substantially flat mirror and a curved mirror is mounted at substantially 45* to the optical axis of the imaging means, the flat mirror reflecting onto one portion of the entrance pupil of the imaging means and the curved mirror on the other, the ratio of radius of curvature of the curved mirror to imaging means entrance aperture being less than 5 and not less than about 1.75, and the radius of curvature of the substantially flat mirror being at least 10 times the radius of curvature of the curved mirror.
 11. A multiple field of view device according to claim 8 in which the reflecting system is a single unitary mirror having a substantially flat portion constituting the flat mirror and a curved portion constituting the curved mirror.
 12. A multiple field of view device according to claim 9 in which the reflecting system is a single unitary mirror having a substantially flat portion constituting the flat mirror and a curved portion constituting the curved mirror.
 13. A multiple field of view device according to claim 10 in which the reflecting system is a single unitary mirror having a substantially flat portion constituting the flat mirror and a curved portion constituting the curved mirror. 